Journal of Experimental Botany, Vol. 42, No. 243, pp. 1225-1231, October 1991
The Effect of a Drying Top Soil and a Possible
Non-hydraulic Root Signal on Wheat Growth
and Yield
A. B L U M 1 ' 3 , J. W. J O H N S O N 2 , E. L. RAMSEUR 2 a n d E. W. T O L L N E R 2
1
Institute of Field Crops, The Volcani Center, P.O. Box 6, Bet Dagan, Israel
2
University of Georgia, Georgia Station, Griffin, GA 30223, USA
Received 2 January 1991; Accepted 13 May 1991
ABSTRACT
Research has shown that plants are adversely affected by a drying top soil and that this effect may be caused, at least partly, by a
non-hydraulic (hormonal) root signal in response to soil water status and/or soil strength. Most studies done to elucidate root
signal effects were limited to the short-term, typically 2 to 3 weeks of plant growth. This study was done to detect a possible nonhydraulic root signal in response to a drying top soil and to measure its effect on plant growth and production from emergence to
maturity.
Wheat (Triticum aestivum L.) plants were established in the growth chamber in soil-filled polyvinyl chloride tubes, 120 cm long
and of an internal diameter of 10-2 cm. Soil was well fertilized and wet to field capacity at emergence when three treatments were
imposed: (1) tubes were watered from the top as needed to eliminate stress (IR); (2) tubes had a constant water table at a soil
depth of 100 to 120 cm, with no applied water (WT); and (3) plants grew on stored soil moisture and were rewatered when water
stress developed (SW). Soil moisture profiles, soil strength, shoot water status and different shoot developmental traits as well as
yield were measured. The upper part of the soil column in WT became dry and hard while roots reached the water table in less
than 30 d. There was no significant difference in midday leaf water potential (LWP) and relative water content (RWC) between
IR and WT, while SW plants developed comparatively lower LWP and RWC. WT as compared with IR, resulted in earlier
heading, smaller flag leaf area, reduced shoot weight, reduced plant height and greater leaf epicuticular wax content. The earliest
effect was seen at 20 d after emergence. Tillers developed after this date had less dry weight in WT than in IR. WT also resulted
in smaller biomass but a greater harvest index than IR, so that both treatments had the same grain yield per plant. Since IR and
WT treatments did not differ in leaf water status, the effect of WT on the plant was intepreted to result from a non-hydraulic
root signal in response to the dry and hard top soil.
Key words: Triticum aestivum, soil moisture, soil strength, root, drought stress, biomass, yield, leaf, tillering, phenology, wax.
INTRODUCTION
When drought conditions develop in the field, the soil
surface becomes dry and hard while deeper soil layers
can contain sufficient moisture for normal plant growth.
Even if deeper roots grow in relatively wet soil, a direct
deleterious effect of the dry and hard soil surface on plant
growth could be suspected. F o r example, in describing
the hierarchical development of wheat tillers, Klepper,
Rickman, and Peterson (1982) observed from their experience that 'it is not unusual to find that wheat plants in
dry, crusted or otherwise unfavourable seedbeds produce
neither the coleptilar tiller (TO) or the first tiller ( T l ) ' .
Masle and Passioura (1987) [and further supported by
3
To whom correspondence should be addressed.
© Oxford University Press 1991
Passioura (1988) and Passioura and Gardner (1990)]
found that wheat seedling shoot growth was negatively
affected by soil strength. This effect could not be
accounted for by either leaf water status, soil phosphorus
content or the amount of seed reserves. They proposed
that the response of wheat to soil strength was conditioned
by some hormonal message induced in the roots when
subjected to high soil strength.
Blackman and Davies (1985) proposed the concept of
a non-hydraulic (hormonal) root-to-shoot communication
of the effect of soil drying. Work by Davies and Zhang
(1987), Zhang and Davies (1989aA 1990) and Zhang,
1226
Blum et al.—Drying Soil, Root Signal and Wheat Growth
Schurr, and Da vies, (1987) repeatedly suggested that roots
responded to soil drying by producing a high concentration of abscisic acid (ABA) which was transported to the
shoot causing stomatal closure and retarding leaf growth.
These effects were independent of the shoot water status.
While the direct effect of the hormonal root signal on
leaf growth was generally accepted, the effect on stomatal
conductance was not always in evidence (Saab and Sharp,
1989).
Working with sorghum plants grown in large containers, Ludlow, Sommer, Flower, Ferraris, and So (1989)
pointed out that while root signals can affect stomatal
conductance and leaf growth, they do not necessarily
affect total plant production and yield. This indirectly
supported Kramer's (1988) criticism that while root signals might exist, their effect on plant production under
natural stress conditions may be negligible as compared
with the more pronounced hydraulic effects. Evidently,
more research is needed on the whole plant development
cycle under conditions of root signal induction.
By definition (Ludlow et al., 1989), a root signal is
suspected when roots are exposed to a drying soil and
plant growth is retarded with no effect on plant water
status. Certain researchers (Masle and Passioura, 1987)
arrived at this experimental condition by growing wheat
seedlings in a dry soil and by pressurizing the soil volume
such that leaf turgor was equalized to that of a wellwatered plant. Others (Zhang and Davies, 1989a) grew
maize or sunflower plants in soil columns which were
allowed to dry for a short period (typically 2 to 3 weeks
under glasshouse conditions) to the extent that plants
were not lacking water but were exposed to a drying top
soil. Alternatively, a split-root system was employed,
where half of the roots were exposed to dry soil while
the other half was watered (Zhang et al., 1987; Saab and
Sharp, 1989). Except for the very large containers used
by Ludlow et al. (1989), these methods were evidently
insufficient for maintaining an isolated root signal induction for a long time, and only short-term (2-3 weeks)
effects on the plant were generally reported.
Our purpose was to study the effect of a dry and a
hard top soil and the resulting non-hydraulic root signal
on wheat plant growth and production. This information
is important in linking the cited physiological experiments
with future field studies, as pointed out by Ludlow et al.
(1989). Conditions for a continuous root signal induction
under the effect of a dry and hard top soil without the
development of a shoot water deficit were induced by
growing the plant in a drying soil column while water
was supplied from a deep water table.
MATERIALS AND METHODS
The experiment was performed in a walk-in growth chamber
under 12 h day, 10/20 °C night/day temperatures, 580 to
600 (umol m~2 s~' of PAR (photosynthetically active radiation)
and 65 to 75% relative humidity. Spring wheat (Triticum
aestivum cv. V652-79) was grown in soil in specially designed
polyvinyl chloride tubes, 120 cm long and of an internal diameter
of 10-2 cm. Each tube was cut longitudinally into two halves
which were then held together by tape and sealed with silicon
rubber. Each tube was fitted with a flange serving as a bottom.
A 6-0 0 mm hole was drilled in the wall, 3-0 cm above the
bottom.
An Appling coarse sandy loam (clayey, kaolinite Typic Hapludult) top soil was air-dried (to — 8-2 MPa of soil water potential),
sieved and mixed with 13-13-13 fertilizer at the rate of 0-28 g
per 1 kg of soil. This soil is considered medium to high in K
and P content and this rate of fertilization is equivalent to the
recommended rate for a high yieldng crop of wheat. A sample
of the mixture was then taken for the determination of the soil
moisture desorption curve by the standard pressure-plate technique. Tubes were filled with the soil to a bulk density of
1-4 gem - 3 . In order to measure soil moisture content by the
time-domain reflectometry (TDR) method (Dasberg and Dalton,
1985; Topp and Davis, 1985; Dalton and Van Genuchten, 1986)
9-0 cm long, 3-1 0 mm stainless steel probes were inserted
horizontally through the tube wall at soil depths of 5-0 cm
and 55 cm. At each depth a pair of probes (spaced at 5-0 cm)
was inserted in parallel so that 5-5 cm of the probe length was
exposed to the soil. A Tektronix model 1502 cable tester was
used as readout. Soil moisture content was calculated from the
readings after calibration against actual soil moisture content
(see below).
Five seeds were planted in each tube to a depth of 2-0 cm
and tubes were irrigated throughout to field capacity with full
strength Hoagland's nutrient solution, which served to fortify
the nutrient status of the soil. Upon emergence, seedlings were
thinned to two per tube. Three water regime treatments were
then imposed. An 'irrigated' treatment (IR) consisted of tubes
irrigated with tap water from the top of the tube twice a week
to replenish water used as estimated by TDR measurements. In
a 'stored water' treatment (SW) plants grew on stored soil
moisture for a period of 57 d after which they were watered as
in the 'irrigated' treatment. Only in this treatment the top soil
was covered upon thinning with a 2-0 cm deep layer of vermiculite, to reduce soil evaporation. A 'water table' treatment (WT)
consisted of tubes standing in cans of water to a depth of 20 cm.
The experimental design was full randomization with 8 replications.
On 21, 31 and 42 d after emergence one tube from each
treatment was disassembled to expose the soil column. Soil
samples (root material removed) were taken with a cork borer
from the space between the two TDR probes (at 5-0 and 55 cm
of soil depth) as well as at depth of 105 cm and were dried to
determine soil moisture content. These values were correlated
(over all sampled tubes) with TDR measurements taken on the
same day in order to develop a calibration curve for TDR. The
soil column with the visible roots was photographed using a
commercial colour slides film. Each slide was projected on a
white paper and the roots were traced. The traced root image
was then scanned electronically and the captured image was
processed for graphic presentation.
Top soil strength was estimated by measuring its impedance
to a penetrating 3-1 0 mm stainless steel probe on a universal
testing machine (Instron model 1132, 2 kg cell). The rate of
penetration was 8-0mms _1 to a soil depth of 10 cm and
impedance was recorded as a function of soil depth. Two
impedance measurements were performed on each tube before
it was disassembled for soil sampling.
On 21, 30, 42, and 57 d after emergence plants were sampled
for midday leaf water potential (LWP) and midday relative
Blum et al.—Drying Soil, Root Signal and Wheat Growth
water content (RWC) measurements. On each date one fully
expanded leaf was sampled from the main stem or the first tiller
of each of four tubes per treatment. The leaf was divided
longitudinally into two strips. LWP was measured in one strip
with the pressure chamber and RWC was measured on the
other strip, using a 4h rehydration period at 10 °C. At 57 d
after emergence, all leaf strips used for LWP measurement were
analysed for epicuticular wax content by the colorimetric method
of Ebercon, Blum, and Jordan (1977).
Each tiller was labelled with a plastic ring marked with the
date of appearance. Each tiller was similarly labelled again for
heading date. Upon heading, flag leaf area for each main stem
was estimated by linear measurements. Main stem height, as a
representation of plant height, was measured from the ground
to the base of the ear. At maturity all plants were harvested
above-ground. Shoot dry matter and total grain number and
weight were determined for each tiller. An account was then
made of tiller number per tube and their respective order of
appearance, heading date, shoot weight, grain number, grain
yield and mean kernel weight.
RESULTS
Soil and roots
Soil impedance to a depth of 10 cm tended to increase
with time in all treatments (Fig. 1). However, soil impedance on all dates and depths measured was much larger
in the 'water table' (WT) and the 'stored water' (SW)
ttreatments than in the 'irrigated' (IR) treatment. On the
first two dates, soil impedance at medium soil depth was
lower in SW than in WT, most likely because of the
surface mulching in the former. This difference disappeared at 42 d after emergence (DAE). The slight increase
in impedance with time in IR could perhaps be ascribed
to an increase in root density at the 0 to 10 cm soil
horizon (Fig. 2).
A significant correlation CR2 = 0-91) was apparent
between soil water potential and soil impedance at a soil
depth of 5-0 cm (where soil moisture data were available).
The relationship took the form of 7=8-33 X0'357, where
Y was soil impedance and X was soil water potential,
both in megapascals. This relationship was used to calcu-
Day 21
late soil impedance at soil depths of 55 and 105 cm, which
were sampled gravimetrically for soil moisture content.
At all sampling dates and soil depths, soil moisture
content was high and soil impedance was low in IR
(Fig. 2A) as compared with WT (Fig. 2B) and SW
(Fig. 2c). Between 21 and 42 DAE, both WT and SW
lost appreciable amounts of soil water which, consequently, resulted in a pronounced increase in soil impedance to a depth of 55 cm or greater. At 42 DAE, soil
moisture at 55 cm soil depth was lower (significant at
P<0-05) in SW than WT, probably because in the latter
treatment plants extracted water from the water table
(Fig. 2B; see root image) rather than from soil at 55 cm
depth. The SW treatment was rewatered at 57 DAE,
when plant water stress was evident (see below).
The shoot
In all treatments, both midday leaf water potential
(LWP) and relative water content (RWC) (Fig. 3) generally decreased with time, possibly as a consequence of
the increase in leaf area and water requirement per plant.
On all dates measured, both LWP and RWC were statistically (P<0-05) the same in IR and WT, apparently
because plants in WT were extracting sufficient moisture
from the water table and/or the soil above it. From 42
DAE, SW resulted in greater plant water deficit than IR
or WT (Fig. 3), until it was watered at 57 DAE.
It should be noted that Fig. 4 describes the results on
a per tube basis, namely per two plants. Averaging these
cumulative data per plant would have reduced their
resolution. On the other hand, the final values reached
in Fig. 4 are presented as means per plant in Table 1.
Beginning on 11 DAE, tillers accumulated in a linear
fashion at the same rate in all treatments (Fig. 4A).
Treatments did not differ significantly in total tiller number per plant at harvest (Table 1), which averaged 13-3 in
SW and 15-4 in IR. Total ear number per plant was 13-6
Day 42
Day 31
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Impedance (MPa)
FIG. 1. Soil impedance profiles to a depth of 10 cm on three dates, as affected by three treatments: fully irrigated, stored soil moisture and water
table. Horizontal bars are standard errors of mean, which are typical for all data presented.
Blum et al.—Drying Soil, Root Signal and Wheat Growth
31 42
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FIG. 3. Midday leaf water potential (top) and relative water content
(bottom) on four dates as affected by three treatments: fully irrigated,
water table and stored soil moisture. Vertical lines represent l.s.d. at
P<0-05.
in IR, indicating a tiller mortality rate of 11-4% under
well-watered conditions (Table 1). Ear number was significantly reduced in WT and SW as compared with IR,
indicating tiller mortality rates of 39-5% and 28-3%,
respectively (Table 1). Ear number was statistically the
same for WT and SW, but tiller mortality rate was
significantly greater in WT than in SW.
The contribution of consecutive tillers to cumulative
shoot dry weight (less grain) per plant increased linearly
in IR (Fig. 4B), to a maximum of 20-0 g (Table 1). Shoot
dry weight in WT and SW reached a maximum of only
13-7 g and 12-9 g, respectively, because tillers appearing
after 20 DAE were relatively lighter in these treatments
than in IR (Fig. 4B). Differences among treatments in
FIG. 2. Soil water potential with depth on three dates (inset: the
respective estimated soil moisture impedance) and the root image on
the surface of the exposed soil column for two dates, (A) Fully irrigated;
(B) water table (shaded area denotes the water table); (c) stored soil
moisture. Horizontal bars are s.e. of mean, drawn only where not
obscured.
Blum et al.—Drying Soil, Root Signal and Wheat Growth
Irrigated
A Stored
D Water Table
01
.0
E
D
kernel number per ear and mean kernel weight (Table 1).
Harvest index was significantly greater in WT and SW
than in IR, mainly because of respective differences in
shoot dry weight.
Although IR and WT did not differ in plant water
status up to 57 DAE, epicuticular wax content of flag
leaves at 57 DAE was significantly greater in WT than
in IR (Table 2) and it was the greatest in SW. This was
visually noticed by the bluish appearance of the leaf
laminae and of waxy bloom on the leaf sheaths in WT
and SW.
In spite of the constant temperature and light regime,
the number of days between the appearance of a tiller
and its heading was reduced in late appearing tillers
(Fig. 5). Data for SW are not presented because they
were too scattered, probably as a result of the water
applied to this treatment at 57 DAE. This watering
occurred after all tillers appeared but before they headed.
On the average, heading was earlier in WT than in IR
by 5-8 d. It commenced in WT at 68 DAE.
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30
40
Date of Tiller Appearance
(Days After Emergence)
FIG. 4. The contribution of consecutive tillers to cumulative tiller number per tube (A); cumulative shoot dry weight (less grain) per tube (B)
and cumulative grain yield per tube (c), as determined at maturity
under the effect of three treatments. Each tube contained two plants.
total above-ground biomass per plant followed those for
shoot weight (Table 1). Main stem flag leaf area was
significantly smaller in WT and SW than in IR (Table 1)
and it was significantly the lowest in SW. Mean plant
height was significantly reduced from IR in both WT and
SW, by about 10 cm (Table 1).
The contribution of consecutive tillers to cumulative
grain yield per plant (Fig. 4c) was similar in all treatments
for tillers formed up to about 20 DAE. Yield became
somewhat greater in WT and SW than in IR after 20
DAE because the late-formed ears were relatively heavier
in the former treatments. However, final grain yield was
statistically the same in all treatments (Table 1), because
more ears were formed in IR than in WT and SW
treatments. Treatments did not differ significantly in mean
DISCUSSION
The water table treatment created a soil environment
whereby the shallower roots were exposed to a hard dry
soil in the top 40 and 60 cm of soil, while deeper roots
were evidently extracting sufficient moisture from the
water table. The irrigated (control) treatment resulted in
a thoroughly wet soil profile with low impedance, as
achieved by normal and frequent watering from the top
of the tube. The mean difference between IR and WT in
the impedance of the top 10 cm of the soil, where penetrometer measurements were actually recorded, was about 7
to 8 MPa. For a period of at least 57 DAE, WT and IR
did not differ in leaf water status, as estimated by two
criteria: LWP and RWC. Mean midday values of LWP
were used also by others in experimental evaluations of
non-hydraulic root signal effects (Saab and Sharp, 1989;
Zhang and Davies, 1989). The error variance for these
two measurements here was quite small, as seen in the
respective small l.s.d. values (Fig. 3), indicating reliable
sampling and measurement procedures and good conditions for resolving small differences between treatments.
Except for the greater waxiness of the leaf laminae and
sheath in WT, no water stress symptoms (such as leaf
rolling or accelerated leaf senscence) were visually noticeable. When water stress was imposed under SW, both
LWP and RWC were affected, until water was applied at
57 DAE. For the first 57 DAE, SW, therefore, served as
a second control, where plant development could be
affected by both a hard top soil and plant water stress.
Slight leaf rolling and a change of leaf colour as typical
symptoms of water stress were noticeable in SW (but not
in WT) before water was applied at 57 DAE.
Plants under WT treatment did not show any visual
symptoms of nutrient deficiencies whatsoever, and the
1230
Blum et al.—Drying Soil, Root Signal and Wheat Growth
TABLE 1. Mean components of plant growth and yield under the effect of three treatments
Values are expressed in absolute values for all treatments and as per cent of the controls (irrigated treatment) for the two stress treatments.
Variable
Unit
Irrigated
(control)
Water table (I)
Value
Total tillers/plant
Total ears/plant
Tiller mortality rate
Shoot dry weight
Biomass/plant
Flag leaf area
Plant height
Grain yield/plant
Kernels/plant
Kernel weight
Harvest index
Days of heading
number
number
per cent
g
g
cm 2
cm
g
number
mg
gg"1
number
15-4
13-6
11-4
20-0
31-9
63-9
75-1
11-9
426
28-6
0-37
82-8
15-2
9-2
39-5
13-7
25-2
48-4
65-2
11-4
407
27-5
0-45
77-1
% control
98-9
67-5"
—
68-8"
79-0"
75-7°
86-8"
96-2
95-6
96-3
120-8°
93-0"
Stored water (II)
Difference
Value
I and IIb
13-3
9-5
28-3
12-9
24-8
38-2
64-9
12-0
417
28-7
0-48
62-1
% control
86-2
69- T
—
64-6°
77-9"
59-8°
86-4a
100-4
97-9
100-4
128-9"
74-9"
N.S.
N.S.
Sig.
N.S.
N.S.
Sig.
N.S.
N.S.
N.S.
N.S.
N.S.
Sig.
" Significantly (/><0-05) different from 100.
' Sig.: significant difference; N.S.; non-significant difference, at P<0-05.
T A B L E 2. Epicuticular wax content of fully
wheat at 57 DAE, under three treatments
expanded
Treatment
Epicuticular wax
(mg cm ~ 2)
Irrigated
Water table
Stored water
36-9 a"
54-4 b
81-1 c
leaves of
" Values followed by the same letter are not significantly different at
P < 0 0 5 , according to Duncan's multiple range test.
TO
(y = 69.2 - 0.31 x Ft2 = 0.58) • Irrigated
(y = 66.5 - 0.45x R2 = 0.74) • Water Table
CO
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O
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Q.
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<
E
o
co
co
Q
Date of Tiller Appearance
(Days After Emergence)
FIG. 5. Days from tiller appearance to its heading in relation to date
of tiller appearance in plants grown under full irrigation or on a water
table.
very pronounced effects of this treatment can not be
ascribed merely to a nutrient deficiency. Furthermore, the
pronounced increase in leaf epicuticular wax content, as
seen here in WT, has never been ascribed to a nutrient
deficiency of any sort and it is recognized as a result of
plant growth under increasing soil moisture stress (Richards, Rawson, and Johnson, 1986).
Considering the data for both soil and plant conditions,
differences in plant responses between WT and IR were
most likely a result of a non-hydraulic effect of the
strength and/or the dryness of the top layer of the soil.
This soil condition caused an increased content of leaf
epicuticular wax, reduced ear number per plant due to
promoted tiller mortality, decreased shoot weight, reduced
plant height and earlier heading. Grain yield per plant
was not affected by this soil condition because the reduction in ear number was compensated for by an increase
in ear weight of later tillers. Mean kernel number per ear
and mean kernel weight were not affected by this
treatment.
An increase in tiller mortality due to a hard soil surface
could be caused by soil resistance to tiller root penetration,
resulting in poor establishment of the latest tillers when
soil surface was the hardest. However, the reduced tiller
shoot weight, early heading, smaller plant height and the
greater load of epicuticular wax cannot be explained
simply by a physical contact of the shoot with the hard
soil surface. The only possible explanation appears to be
that these effects resulted from a root signal induced by
root exposure to a hard and dry top soil. The earliest
possible effect of an apparent root signal in this experiment could be seen at about 20 DAE. Tillers which grew
after this date had less dry weight than tillers in the
control.
The addition of a soil moisture deficit component to
the system, as imposed by SW in comparison with WT,
caused a plant moisture stress to develop (which was
alleviated by watering at 57 DAE). Plant moisture stress
further reduced flag leaf area and increased epicuticular
wax content beyond that recorded in WT treatment. The
decrease in tiller mortality under SW as compared with
WT is a computational result from the fact that the two
Blum et al.—Drying Soil, Root Signal and Wheat Growth
treatments produced the same number of ears with fewer
tillers (non-signficant) in SW. Yield and its components
were unaffected by water stress in SW, probably because
water stress was not allowed to increase, having been
terminated by watering at 57 D A E (before heading).
The apparent non-hydraulic root signal sensed here
was induced under a comparatively realistic situation. In
the field, roots may grow within a soil profile that is dry
and hard at the t o p and wetter at deeper layers, typical
of crops grown under dryland conditions. A n o n hydraulic root signal under such conditions may have a
role in reducing leaf area and shoot size (as seen here) in
order to economize water-use, even before shoot water
deficit develops. In this sense, the plant is being prepared
for the forthcoming water deficit. As Ludlow et al. (1989)
has argued, whether this plant response is beneficial or
disadvantageous t o crop production largely depends on
the nature of the ensuing water regime. If plant water
deficit indeed develops, then this response may offer an
advantage in balancing water-use against water availability. On the other hand, if the soil profile is recharged by
rainfall or irrigation before plant water deficit develops,
a root signal may result in an unnecessary loss in plant
production.
In our experimental system, the reduction in plant size
under the possible effect of a root signal could not have
had any feedback on plant water status, because deeper
roots were always immersed in water a n d soil moisture
supply did not recede with time as would be expected in
the field. In this sense, the situation was unrealistic. While
the suggested root signal effects were prominent for plant
size, biomass, leaf area, growth duration and plant surface
waxiness, more research is needed t o quantify the effect
of such modifications on yield under the dynamics of the
soil-plant-atmosphere continuum in the field.
ACKNOWLEDGEMENTS
This research contribution was supported by state a n d
Hatch funds allocated to the Georgia Agriculture Experiment Station. The preparation of the soil moisture desorption curve by D r L. M . Shuman is gratefully
acknowledged.
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